The present invention relates generally to a power flow control system for controlling the power flow level over a transmission line, and more particularly to a power flow control system and a method for reversing the direction of power flow through the transmission line.
A typical power transmission system is shown as a single line schematic diagram in FIG. 6. FIG. 7 shows a phasor diagram of the power parameters of the FIG. 6 system. For the purposes of discussion herein, a "positive power flow" refers to a power flow through a transmission line L from a first voltage source V.sub.1 toward a second voltage source V.sub.2, and a reverse or negative power flows in the opposite direction. This positive power flow direction is also illustrated in FIG. 6 by the direction of the arrow corresponding to a line current I flowing through the transmission line L. The transmission line L is an alternating current (AC) line having an impedance Z.sub.L which is dominantly inductive or positive.
Power flow P through the transmission line L is, to a good approximation, governed by the equation: EQU P=(V.sub.1 V.sub.2 sin .delta..sub.12).div.(-Z)
In this equation, V.sub.1 and V.sub.2 are the two line-end voltages shown in FIGS. 6 and 7, .delta..sub.12 a phase angle between the V.sub.1 and V.sub.2 voltages, and Z is the net series impedance of the line L.
One earlier manner of controlling power flow over the transmission line L controls the net series impedance Z of the line. Since the natural impedance of a transmission line is inductive (Z.sub.L), one or more series capacitors having a capacitive impedance Z.sub.C are sometimes used to decrease the inductive impedance Z.sub.L of the line. Such series capacitors are switched in and out of series with the transmission line in steps to vary the net inductive impedance Z of the transmission line L.
Under the present state of the art, rather than a stepwise insertion, the value of the series capacitor can also be controlled smoothly by coupling the series combination of a reactor and a thyristor switch (not shown) in parallel with the capacitor. By controlling the firing angle of the thyristor, the apparent impedance of the capacitor can be smoothly varied. For economic reasons, combinations of stepped and variable capacitor assemblies have sometimes been used to accomplish the required range of impedance. Thus, the series capacitance may be inserted in a stepped variable or a gradually variable fashion, or in a combination thereof.
These earlier systems are limited to controlling the level of power flow in only a single direction, specifically, from V.sub.1 to V.sub.2 when the V.sub.1 voltage is leading the V.sub.2 voltage, as shown in FIG. 7. In these earlier systems, the only way to reverse the direction of power flow from V.sub.2 to V.sub.1 is to reverse the phase angle .delta..sub.12, so that the second voltage V.sub.2 leads the first voltage V.sub.1, shown in dashed lines in FIG. 7 as vector V.sub.1 '. The only practical manner of reversing the phase angle, shown as angle -.delta..sub.12, is to make significant changes in the power generation schemes of the voltage sources V.sub.1 and V.sub.2. These major generation changes are quite impractical and not easily satisfied in complex power systems.
One severe limitation of the earlier system of FIG. 6 is that the phase angle .delta..sub.12 drifts back and forth, such as from +.delta..sub.12 to -.delta..sub.12 as shown in FIG. 7. This unpredictable drifting of the phase angle leads to random and undesired changes in the power flow direction.
Another earlier proposed system includes a phase angle regulator. However, these regulators are expensive, and have high losses, as well as other disadvantages. Moreover, phase angle regulators are not cost effective for many applications.
Another system proposed for controlling power flow uses high voltage direct current (HVDC) equipment (not shown) coupled to the transmission line L. In an HVDC implementation, power flow is controlled independent of the value of the phase angle .delta..sub.12. A significant drawback to the HVDC implementation is its expense, in terms of both initial installation and operational costs, so the HVDC implementation is simply not cost effective for many applications.
Thus, a need exists for an improved power flow control system, comprising an apparatus and a method, for selectively controlling the direction of power flow over a transmission line, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.